A Scientific Consensus on Global Warming

By Donella Meadows

–October 11, 1990–

Is there a greenhouse effect or isn’t there? In the United States that question has become political. Some people think global warming is an enormous threat; therefore we should change our polluting ways. Some think that is pernicious poppycock. Several of each persuasion whisper into the ear of the President.

The discussion should not be political, of course. It should be scientific. Most of the scientists I know are very worried about global climate change. But there are a few loud doubters, who are of great use to the political doubters.

Knowing that tremendous policy issues are at stake, the scientists are trying to come to as clear an understanding as possible about global warming. Their most broadly based and carefully worked-out statement has just been released. It is a strong document. Says Science magazine: “There’s virtual unanimity among greenhouse experts that a warming is on the way and that the consequences will be serious.”

The new report comes from Working Group 1 of the Intergovernmental Panel on Climate Change (IPCC), coordinated by the World Meteorological Organization and the United Nations Environmental Program. Working Group 1 started with 170 scientists from 25 countries attending 12 workshops around the world. Thirty four authors wrote up the conclusions from those workshops, which were then reviewed by another 200 scientists.

They had little problem coming to agreement. Only the Americans were surprised by this — because only in the United States has the greenhouse problem become politicized. Says British climatologist Christopher Folland, “In America a few extreme viewpoints have taken center stage. There are none like that elsewhere.”

Here is what the IPCC report says in its summary, point by point. Except where there are quotation marks, I have rephrased each point to simplify scientific language.

“We are certain of the following:”

“There is a natural greenhouse effect which already keeps the Earth warmer than it would otherwise be.”

Emissions resulting from human activities are increasing the atmospheric concentrations of greenhouse gases. These increases will warm the Earth still further.

“We calculate with confidence that:”

“Some gases are potentially more effective than others at changing climate…. Carbon dioxide has been responsible for over half the enhanced greenhouse effect in the past, and is likely to remain so in the future.”

Many of the greenhouse gases we are emitting change only slowly in the atmosphere. Once they are increased, they will stay that way for centuries to come.

To stabilize the concentrations of the greenhouse gases at today’s levels, we would have to reduce our emissions by over 60 percent, except for methane, which we would have to reduce by 15-20 percent.

“Based on current models we predict:”

If there are no changes in greenhouse gas emissions, the global mean temperature will increase by about 0.5 degrees F. per decade. (That looks like a small change, but to put it in context, the terrible summer of 1988, the warmest in history, was just 0.6 degrees F. warmer than the 1950-1975 global average.)

Reduced emission levels can slow these increases in temperature.

The land will warm more rapidly than the ocean, and northern latitudes will warm more than the tropics.

If there are no changes in emissions, there will be a global mean sea level rise of 2.5 inches per decade over the next century.

The timing, magnitude, and regional patterns of climate change are still uncertain because of incomplete understanding of the sources and sinks of greenhouse gases and the role of clouds, oceans, and polar ice sheets. “We cannot rule out surprises.”

“Our judgement is that:”

Global mean air temperature has increased by 0.5 to 0.9 degrees F. over the past 100 years. The five warmest years on record have been in the 1980s. “These increases have not been smooth with time nor uniform over the globe.”

The warming observed is consistent with an enhanced greenhouse effect, but it is still within the range of natural variation. It does not yet prove the validity of the climate models, nor does it disprove them.

Rapid changes in climate will change ecosystems. “Some species will benefit while others will be unable to migrate or adapt fast enough and may become extinct.” The changes in the ecosystems may further change the climate, in a vicious cycle that releases still more greenhouse gases and speeds up the warming.

A few weeks ago I was in a meeting at which strategic planners from multinational corporations — many of them oil and car companies — were discussing these results. One of them said that the evidence was quite sufficient to justify taking action to prevent further global warming. We took a straw vote to see how many agreed. With the businessmen, as with the scientists, it was nearly unanimous.

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About The Donella Meadows Project

The mission of the Donella Meadows Project is to preserve Donella (Dana) H. Meadows’s legacy as an inspiring leader, scholar, writer, and teacher; to manage the intellectual property rights related to Dana’s published work; to provide and maintain a comprehensive and easily accessible archive of her work online, including articles, columns, and letters; to develop new resources and programs that apply her ideas to current issues and make them available to an ever-larger network of students, practitioners, and leaders in social change. Read More

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